[flang] Fix build problem with clang

[flang] Fix build problem with clang

Moving one of the `Pre(ImplicitStmt)` functions up to
`DeclarationVisitor` seemed to cause clang to complain that the `Pre`
call from the `Walk` function was ambiguous. So this change moves
it back to `ResolveNameVisitor`.

Also, remove an unused variable that was causing a warning.

Original-commit: flang-compiler/f18@766d000e2507d92b7774ad2c37732bafccc7cbcb
Reviewed-on: https://github.com/flang-compiler/f18/pull/299